In order to offer the person sitting in a chair good seating comfort, in the case of high-quality office chairs, it is known that the seat and the backrest (seatback) can be movably coupled with one another via a synchronizing mechanism in such a way that an adjustment of the backrest also has as a consequence a simultaneous and dependent adjustment of the seating surface. As a result of this synchronized adjustment of the seat and the backrest, the chair is adapted to the ergonometry of the person using the chair.
During adjustment of the backrest of such office chairs, the leaning force of the person using the chair is opposed to the readjusting force of a readjusting apparatus that simultaneously supports the seat, which is joined with a seat support in a swiveling fashion, in opposition to a swinging movement. In this regard, the readjusting force acting against the leaning force of the person using the chair must be greater in the case of large and heavy persons, due to the greater body weight, and must increase even more with increasing backward inclination of the backrest and in accordance with a lengthened lever arm, whereby, however, this increase in force should still remain dependent on the weight of the person using the chair.
In order not to burden the person using the chair with the adaptation of the adjusting apparatus to his body weight, and in order to eliminate resulting erroneous adjustments, various office chairs have already been created in which the adaptation of the readjusting force to the leaning force of the person using the chair is carried out automatically depending upon the person's body weight.
From German utility model DE-GM 86 14 185.6 an office chair is known, whose seat is joined by means of a front and a rear parallelogram link rod to a fixed seat support that is attached to a lower part of the seat. The seat of this previously known office chair is movably coupled with the backrest by means of a synchronizing mechanism comprising additional swiveling link arms in such a way that during an adjustment of the backrest, the seat is lowered from its normal position to a greater extent in the rear than in the front, and carries out a movement towards the rear following the backrest. As this is taking place, the seat is supported by means of a readjusting apparatus that acts in opposition to the swinging movement and that consists of two readjusting springs that are more or less strongly prestressed, depending upon the weight of the person using the chair. By means of the readjusting apparatus and its readjusting springs, it is possible to achieve a movement of the backrest, together with the synchronized movement of the seat that is coupled with it, that takes place more or less with little effort.
This previously known office chair has a relatively complex mechanical design characterized by a large number of linking points and movable link arms in order to coordinate the sequence of movements between the seat and the backrest. As a result, there exists the problem of creating, in particular, an office chair of the type mentioned at the outset, in which the readjusting force of the backrest is automatically adapted to the weight of the person using the chair, and which increases with increasing inclination of the backrest, but is distinguished by a simple mechanical design.